Field of the Invention
The present invention relates to an image capturing apparatus and a control method therefor, and in particular to a technique to generate a high-dynamic-range image.
Description of the Related Art
A technique called high-dynamic-range (HDR) composition is known in which a dynamic range of an image is increased by compositing a plurality of images captured under different exposures (underexposure and overexposure). With this technique, correctly-exposed portions of various images are composited so as to suppress blown-out highlights and blocked-up shadows and obtain an image presenting preferred tones from a dark region to a bright region.
The HDR composition technique is applicable not only in capturing still images, but also in capturing video. There are configurations for performing HDR composition in capturing video (HDR video capture), one known example being a configuration in which video is captured while applying different exposures (underexposure and overexposure) to frames of the video in an alternating manner.
Japanese Patent Laid-Open No. 2012-049600 discloses a technique to apply white balance control to video frames, and perform HDR composition using images to which the white balance control has been applied. On the other hand, Japanese Patent Laid-Open No. 2009-278524 discloses a technique to perform image capture consecutively while changing the exposure, and perform HDR composition using a plurality of temporally neighboring images. In processing related to HDR composition, processes for temporally neighboring images overlap; in this way, the processing result of an image that is processed first is reused in the next HDR composition, and hence the processing is executed efficiently.
With the technique disclosed in Japanese Patent Laid-Open No. 2012-049600, HDR composition processing is delayed until processing necessary for the white balance control, such as extraction of color information, estimation of light source information, and setting of a white balance gain, is completed for all frames. Therefore, in a case where HDR composition is performed using images of two frames with different exposures, a frame rate is reduced by half. On the other hand, with the technique disclosed in Japanese Patent Laid-Open No. 2009-278524, a frame rate can be practically maintained due to reuse of the process for one of temporally neighboring images. However, as the image processing result of one image is reused in HDR composition, in a scene where colors and luminances change with time, deterioration in the image quality associated with colors and luminances is expected to occur in the form of, for example, a tone jump due to the failure to apply appropriate image processing.